The invention relates generally to crystals that allow selected wavelengths of light to pass therethrough, and more particularly, to optical band-pass or rejection filters that operate in the ultraviolet (UV) spectrum.
A variety of optical devices utilize optical band-pass filters or rejection filters, which serve to reject unwanted wavelengths and permit only wavelengths belonging to a selected spectral band to pass therethrough. Crystals useful in rejection band-pass filters in the UV portion of the spectrum can be used for various applications, including solar-blind optical systems and missile warning systems. The following references discuss crystals and/or UV light filters and are incorporated herein by reference: M. A. Res et al., xe2x80x9cBand-Pass Filters for Use in the Visible Regionxe2x80x9d, Appl. Opt., 16 (1977) 1908-1913; W. Luo and F. Zhu, xe2x80x9cLongwave Ultraviolet Glass Filter Without Transmission of Infraredxe2x80x9d, Appl. Opt., 29 (1990) 4549-4550; U.S. Pat. Nos. 5,788,765 and 5,837,054; M. H. L. Pryce et al., xe2x80x9cLow-Temperature Absorption of Nickel Fluorosilicate Crystalsxe2x80x9d, Philos. Mag., 10 (1964) 477-496; G. H. Dieke et al., xe2x80x9cThe Spectra of the Doubly and Triply Ionized Rare Earthsxe2x80x9d, Appl. Opt., 2 (1963) 675-686; D. J. Ehrlich et al., xe2x80x9cUltraviolet Solid-State Ce YLF Laser at 325 nmxe2x80x9d, Opt. Lett., 4 (1979) 184-186; S. L. Baldochi, K. Shimamura, K. Nakano, N. Mujilatu and T. Fukuda, xe2x80x9cCe.-doped LiYF4 Growth Under CF4 Atmospherexe2x80x9d, J Cryst. Growth, 205 (1999) 537-542.
The light emitted from the exhaust of a jet engine of a missile comprises a broad spectrum, ranging from the infrared (IR) to the UV. Thus, a UV filter is needed to enable the system to distinguish the source of UV light from other natural sources of UV energy, such as the sun, moon, and stars.
Crystals of hydrated nickel salts and colored UV glasses are used conventionally in the construction of band-pass UV filters. The hydrated nickel salts that can be used for UV filters include nickel sulfate hexahydrate (chemical formula NiSO4.6H20), nickel hexafluorosilicate hexahydrate (chemical formula NiSiF6.6H2O) and potassium nickel sulfate hexahydrate (chemical formula K2Ni (SO4)2.6 H2). In the UV part of the spectrum, these materials are transparent at wavelengths less than 300 nm and block UV light having a longer wavelength. In UV band-pass filters, crystals of hydrated nickel salts can be used to reject UV wavelengths greater than 300 nm.
It is advantageous for an optical system used to detect the UV signal characterizing the jet engine of a missile to operate at various climate conditions, including those associated with elevated temperatures, which might be experienced in the desert. Also, temperatures of 85xc2x0 C. or higher and low relative humidity conditions can be reached when aboard an aircraft, armored vehicle or a ship, as a combined result of both solar heating and engine-generated heat. Thus, the crystal should have not only proper spectral transmission but thermal stability. Unfortunately, the main drawback of the hydrated nickel salts is their low thermal stability. Upon heating, these materials start to lose their water of crystallization at temperatures as low as 60 to 65xc2x0 C. for nickel sulfate and at 90 to 110xc2x0 C. for the other two salts.
Standard UV glasses are available from several commercial manufacturers, including Schott (UG-5 and UG-11) and Hoya (U-360). Fundamentally, UV glass represents a phosphate glass matrix colored by additives of various metal oxides, such as CuO, CoO, TiO2, NiO, Cr2O3, and V2O5. UV glasses are transmissive at desired UV wavelengths and highly absorptive at larger wavelengths. However, glass filter elements also have drawbacks. Although stable and rugged, they do not have sharply defined narrow absorption bands characteristic of crystals. Typically, absorption bands of a colored glass are broad and their boundaries are gradual with a typical slope of 0.1 db of absorption per nanometer. As a result, either the width of the band-pass or the maximum band-pass transmission must be compromised.
Therefore, there is a need for durable UV single crystal filter materials with narrow and sharp absorption bands in the UV portion of the optical spectrum.
Generally speaking, the invention is directed to crystals having good optical transmission between 250 nm and 300 nm, more preferably between 260 nm and 280 nm and absorb strongly at 285 nm and higher, and which are nonhydroscopic and stable at temperatures over 110xc2x0 C. and even up to 250xc2x0 C. for indefinite time periods. The crystals of the present invention are also stable when maintained at up to 50xc2x0 C. and 90% relative humidity for at least 24 hours. An example of such a crystal is Cerium doped YLiF4 single crystals. These properties make YLiF4 crystals doped with Ce3+ excellent materials for use as UV filters in the missile early warning spectral band. Optical devices including missile detection systems are also provided.
Accordingly, it is an object of the invention to provide an improved UV transmission crystal.
It is another object of the invention to provide a UV transmission crystal having high thermal stability.
Another object of the invention is to provide an improved missile detection system.
As to other objects and advantages of the invention, they will in part be obvious and will in part be apparent from the specification and drawings.
The invention comprises the several steps and the product resulting from such steps possessing the features, properties and relation of components which will be exemplified in the products and methods hereinafter described, and the scope of the invention will be indicated in the claims.